Metrohm Vision – Diagnostics - Manual (Page 37)

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9
Wavelength Linearization
9.1 Wavelength Linearization on Model 5000 and 6500
Wavelength scale registration of the instrument is an important characteristic. Metrohm instruments
are designed to provide wavelength response consistent with NIST-defined wavelength standards. To
this end, an internal polystyrene/didymium paddle is inserted into the instrument beam upon
software command, and selected peak positions of each material are used to maintain accuracy. The
peak positions required to achieve correct wavelength registration of the instrument are called
“nominals”. Wavelength Linearization provides long-term wavelength accuracy and precision for all
Metrohm instruments.
Wavelength Linearization Constants
A polystyrene standard (4 peaks) is used in the 1100-2500 nm region, and a polystyrene-didymium
standard (3 peaks) is used in the 400-1100 nm region. Actual peak positions are compared with
nominal peak positions stored in a software table. Differences between nominal and actual peak
positions are calculated, and corrections are made by instrument software, which are applied to all
subsequent sample spectra. The tolerance is +/- 0.3 nm for laboratory instruments, and 0.5 nm for
process instruments. If the test is successful, Vision asks if the operator wishes to apply the
linearization constants to the instrument. Click on OK to send linearization constants to the
instrument.
The Wavelength Linearization procedure can be performed automatically (“Auto-linearization”)
whenever a reference is scanned, to update the wavelength correction at each reference scan. This is
also performed as part of regular Performance Test. To apply Auto-linearization (recommended)
select Configure, Options, then click on Auto-linearization. When this is checked, wavelength
correction will be applied to every reference and sample scan. Auto-linearization adds a slight
amount of time to each scan.
Wavelength Linearization should be performed manually whenever a module is changed on a
laboratory instrument, as well as after a lamp change. Once this has been performed manually,